All the choices seem possible good answers, but D is the correct one:
ADrakensberg Moutains are the extreme south-east part of the "Great Escarpment", they may result an obstacle only on the far coastal zone (they do lack signs of civilization); b. The Great Rift Valley runs in the mid-east zone, but fossils, even 3 million years old, were discovered here, a proof of civilization; c. The Nile River was actually a resourceful water bank for the flourishment of the Egyptian Empire, it wasn't an obstacle for West African expansion per se; D. The Sahara Desert is the main and greatest geographical feature (among these choices) stopping people from West civilized expansion, in fact, only nomads have lived and still live there.
